Highly exposed persistently seronegative individuals provide a valuable opportunity to study immune parameters that may correlate with protection from overt HIV-1 infection. This study will investigate the role of T cell immunity, HLA, and macrophage function in HIV seronegative sex workers of more than four years duration from the truck stops in KwaZulu-Natal and HIV seronegative men who are repeat attenders at the Durban sexually transmitted diseases clinic. The frequency and durability ofHIV-1 antigen-specific CD8+ and CD4+ T cell responses will be investigated and correlated with coital frequency in these individuals. T cell reponses will be assessed using a combination of the IFNg ELISPOT assay, intracellular cytokine staining by flow cytometry, functional kill and proliferation measurements. This approach will identify CTL epitopes and explore CD4+ T cell responses in relation to the breadth and magnitude of CTL. The possibility of latent HIV-1 infection in CD4+ T cells in highly exposed persistently seronegative individuals who show detectable T cell responses will also be investigated. We wish to determine whether low-level infection may drive T cell immunity. The monocyte/macrophage lineage will also be investigated for the presence of incomplete, defective HIV-1 genomes as well as exploring the phenomenon of stimulation-induced resistance in macrophages isolated from blood and genital tract. Stimulation of isolated monocytes/macrophages with a representative panel of bacterial and viral antigens derived from concurrent sexually transmitted infections will be investigated. We aim to assess whether stimulation-induced cytokine/chemokine expression can render cells resistant to ex vivo HIV infection. One of the outcomes of this study will be to identify CTL epitopes that show degenerate HLA binding and which are correlated with protection from HIV infection in the epicenter of the global HIV epidemic.